The present invention relates to a convection heat treating apparatus for thermal treatment of a traveling substrate, such as the drying or heat setting of a traveling web of textile material with a treatment gas, and more particularly to a convection heat treating apparatus adapted to control the flow and temperature of the treatment gas for proper treatment of the traveling web.
Conventional convection drying or setting machines generally have application chambers extending over and under the surfaces of a traveling fabric web, a distribution chamber connected to the application chambers with a fan for circulating the treatment gas, and a return flow chamber following the application chambers in the direction of flow of the treatment gas created by the fan. The return flow chamber communicates with the distribution chamber and contains a heating arrangement for reheating the treatment gas for recirculation. The inlets of the application chambers have flow control valves and the distribution chamber generally has at least one by-pass opening leading to the fan and adjacent to the inlet of the application chamber. The flow control valves are moveable to open and close the inlets to vary the flow therethrough by movement between positions opening, closing, and varying the opening of the inlet. The flow control valve of an inlet having a by-pass opening adjacent thereto is generally adapted to open and close the inlet inversely to the opening and closing of the by-pass opening. In this manner, the thermal treatment of the fabric web is controlled, but not without some disadvantages.
The treatment gas in conventional machines is generally heated air, but the circulating air may be any gas used in such machines. With conventional machines which utilize gas to heat the circulation air, the burners must be turned down low when the fabric web is stopped and the circulating air must also be throttled by means of the flow control valves. This decreases the temperature of the treatment gas. When the fabric web is started again the treatment gas usually does not return to the proper temperature "under load" as fast as the possible acceleration of the fabric web to be treated. The result is inadequate setting, streaking of the fabric or other attenuated treatment of a portion of goods.
In the case of machines heated by circulating oil or steam, the treatment gas is also throttled and the energy supply is stopped when the fabric web is stopped. If there is no means to dissipate heat from the treatment gas heaters, the temperature of the surfaces of the machine in the vicinity where heating occurs can increase sharply due to the build up of heat of the heating medium. Therefore, when the machine is restarted, the treatment gas is heated to a temperature beyond the normal value and the result can be overheating and excess setting of the fabric web.
Known prior art convection drying or setting machines are disclosed in British Pat. No. 235,402 and West German Pat. No. 3,336,331. The British patent involves forcing the circulating air by a blower through a radiator into an application chamber. A by-pass is provided to take the radiator out of the path of circulation of the treatment gas. Thus, the treatment gas changes almost instantaneously from heated to cool circulating air. As previously described, when the fabric web is restarted, and the radiator is switched back into the path of circulation, there will be a delay until the treatment gas regains its proper temperature during which time improper treatment of a portion of the goods will result. The West German patent describes a generic machine which has the objective of avoiding the pronounced streakiness which results in sensitive fabrics such as knitted fabrics when the machine is stopped and started without adjusting the temperature of the treatment gas. This patent provides a by-pass opening adjacent each application chamber inlet that communicates directly with the return flow chamber. The by-pass opening thus permits the treatment gas to pass directly into the return flow chamber without impacting upon the fabric web. Each inlet and by-pass opening has a common flow control valve moveable for partial or complete closure of the application chamber inlet inversely to the adjacent by-pass opening. Thus, the temperature of the treatment gas may be maintained at the value set for normal treatment during stoppage of the fabric web and the treatment gas is directed by the flow control valve into the return flow chamber for reheating without coming into contact with the fabric. There is no change in the heating of the treatment gas, however, and it is blown with the full force imparted by the fan against internal machine components not intended to be heated, particularly the roof and especially the side door of the machine. Overheating can eventually occur when the machine is stopped, resulting in adverse treatment of the fabric web.